1. Field of the Invention
The present invention relates to an electronic device having electric wires such as a wire-wound chip coil and to a method of producing such a coil device. The present invention also relates to a method of producing an electronic device having electric wires in an inductance component. More particularly, the present invention relates to a method of producing an electronic device having electric wires including an improved process of connecting end portions of an insulated electric wire wound around a core to electrodes located on the core.
2. Description of the Related Art
FIG. 5 illustrates a conventional chip coil made up of an electric wire 5 wound around a core 1 made of a magnetic material, wherein ends 5a of the wire 5 are connected, via thermo-compression bonding such as a wire bonding process, to respective electrodes 2 located on the core 1. The electrodes 2 are made of a material such as Ag or Agxe2x80x94Pd. When the ends 5a of the wire are connected to the respective electrodes 2, the resultant connecting portions of the wire 5 have raised portions bulging from the surface of the electrodes 2. The bulging shape of the connecting portions causes the chip coil to become unstable when it is mounted on a printed circuit board. That is, the chip coil is mounted unevenly in a slanted orientation or topples over and is separated from the printed circuit board in the worst case. Another problem with the chip coil of this type is that the ends 5a of the wire 5 are exposed directly to air and thus, the ends of the wire are oxidized. This makes it difficult to solder the ends of the wire during the process of mounting the chip coil.
One possible technique of improving the stability of the mounted position is to form recesses 3 as seen in FIG. 6 in both end portions of the core 1 so that the ends 5a of the wire can be placed inside the recesses 3. However, the shape of the core 1 becomes complicated and difficult processes are required to produce such a complicated structure including the recesses 3.
The insulated electric wire 5 generally consists of an electric wire made of metal such as copper whose outer surface is coated with an insulating material such as polyesterimide. The insulated electric wire is connected to the electrodes via, for example, pulse heating. The connection process via pulse heating is described below with reference to FIG. 11.
In FIG. 11, cross sections of the core 51 of the coil device and the electrode 52 located on the upper surface of the core 51 are shown. An end portion of the insulated electric wire 53 is disposed on the electrode 52.
A pressing tip 56 heated at about 500xc2x0 C. is moved down so that the insulated electric wire 53 is pressed against the electrode 52. The electric wire 53a is flattened by the pressure and the electric wire 53a is connected to the electrode 52 via thermo-compression bonding.
In this connection technique, if the electrode 52 is made of metal having a high melting point such as Ag, Cu, or Ni, the insulating coating 53b melts at a temperature lower than the melting point of the electrode 52, and the electric wire 53a and the electrode 52 are directly connected to each other. Another feature of this technique is that the electric wire 53a is flattened by the pressure.
However, although the electric wire 53a on the electrode 52 is flattened, there is still a processing step required on the surface of the electrode 52 and the electric wire 53a. When the coil device is mounted on a printed circuit board such that the surface of the electrode 52 and attached electric wire 53a comes into contact with the printed circuit board, the above-described step can cause the coil device to become unstable or cause the soldered connection to become unreliable.
In many cases, the surface of the electrodes 52 of the coil device is plated with metal having a low melting point such as Sn or solder so that a low-melting-point electrode layer is formed on the electrode 52 thereby ensuring that the electrode can be easily soldered. For example, as shown in FIG. 12, the electrode 52 is produced by coating a silver-filled paste on the surface of the core 51 and baking it so as to form a base layer 52a, then plating the surface of the base layer 52a with Ni thereby forming a Ni-plated layer 52b for protecting the base layer 52a from being eroded by solder, and finally forming an electrode layer 52c of low-melting-point metal which allows the electrode 52 to be easily soldered.
In the case of the coil device having the above structure, when connection is performed with the pressing tip 54 heated at about 500xc2x0 C., the electrode layer 52c is heated by the pressing tip 54 to a temperature higher than the melting point of the low-melting-point metal. In the connecting process, the electrically conductive wire 53a is pressed by a pressure high enough to compress the electrically conductive wire 53a. As a result, the insulating coating 53b and the electrode layer 52c made of the low-melting-point metal at the top layer are both melted into liquid states, and the electrically conductive wire 53a is compressed into a flattened shape.
As a result, in the pressing process using the pressing tip 54, the low-melting-point metal in the liquid state is pushed aside by the insulating coating 53b in the liquid state toward the sides of the electrically conductive wire 53a. In the above process, after the low-melting-point metal is pushed aside by the melted insulating coating 53b, if the melted insulating coating 53b sticks to the pressing tip 54 and is removed when the pressing tip 54 is moved up to its original position, the Ni-plated layer 52b is sometimes exposed in an area A at a side of the conductive wire 53a. If the Ni-plated layer 52b is partially exposed, when the coil device is mounted on a printed circuit board via soldering, a connection failure can occur because the Ni-plated layer 52b has poor solder wettability.
Even in the case where the Ni-plated layer 52b does not become exposed in the area A in FIG. 12 after the low-melting-point metal is pushed aside by the melted insulating coating 53b, if the melted insulating coating 53b sticks to the pressing tip 54 and is removed when the pressing tip 54 is moved up to its original position, a crater 52d is produced in an area on the surface of the electrode 52 where the insulating coating 53b of the electrically conductive wire 53a was present. If such a crater is produced, it becomes difficult to make a good connection in the soldering process.
To overcome the problems described above, preferred embodiments of the present invention provide an electronic device including a wire which is mounted in a stable fashion, firmly connected to an electrode and is very resistant to oxidization and a method of producing such an electronic device.
In addition, preferred embodiments of the present invention provide a method of producing a coil device, capable of connecting an insulated electric wire to an electrode for connection to an outer circuit in such a manner that the insulated electric wire is embedded in the electrode thereby ensuring that the coil device can be connected to the outer circuit in a highly reliable fashion without causing a soldering failure or any problem which occurs in the conventional techniques.
According to a preferred embodiment of the present invention, an electronic device includes an electric wire firmly connected to an electrode located on an insulating base, the electrode including at least a solder barrier layer made of a material with a high melting point and an easy-soldering layer made of a material with a low melting point; and the electric wire is embedded via a thermo-compression process in the easy-soldering layer in such a manner that the resultant structure has a substantially flat surface.
In the electronic device according to preferred embodiments of the present invention, the flattened electric wire is embedded in the easy-soldering layer so that the resultant structure of the electrode has a flat surface including no raised portions extending from the electrode. As a result of this structure, the electronic device can be mounted in a stable fashion on a circuit board without causing the electronic device to be slanted or to topple over.
In the electronic device according to preferred embodiments of the present invention, the surfaces of the easy-soldering layer and electric wire are preferably covered with another easy-soldering layer so that the surfaces of the electrodes become flatter and so that the surfaces of the electrodes are protected from being oxidized.
According to another aspect of preferred embodiments of the present invention, there is provided a method of producing an electronic device, including forming an electrode on an insulating base, the electrode including at least a solder barrier layer made of a material with a high melting point and an easy-soldering layer made of a material with a low melting point disposed on the surface of the solder barrier layer; and pressing the electric wire against the electrode while heating the electric wire and the electrode so that the electric wire and the solder barrier layer are connected together via solid welding and so that the electric wire and the easy-soldering layer are connected together via brazing.
In this production method according to preferred embodiments of the invention, the easy-soldering layer is melted via heating and compressed via pressing so that the electric wire sinks into the easy-soldering layer. As a result, the electrode has a structure having a substantially flat surface in which the electric wire and the solder barrier layer are connected to each other via solid welding and the electric wire and the easy-soldering layer are connected to each other via brazing. This technique makes it possible to connect the electric wire to the electrode in a highly reliable manner. In the case where the electric wire is covered with an insulating coating such as polyesterimide, the insulating coating is melted/vaporized via heating in the above production process. Therefore, no additional process for removing the coating is necessary.
In the connection process described above, it is preferable that the electric wire and the electrode be heated so that the temperature thereof increases in a very short period of time to a value higher than the melting point of the easy-soldering layer and lower than the melting point of the solder barrier layer, and then cooled so that the temperature decreases in a very short time to a value lower than the melting point of the easy-soldering layer. Although the time periods of heating, sustaining the elevated temperature, and cooling vary slightly depending on the diameter and material of the electric wire, each time period is preferably set to a value shorter than about a few seconds and more preferably shorter than about 1 second.
According to still another aspect of preferred embodiments of the present invention, there is provided a method of producing a coil device, the coil device including a core made of an insulating material, an insulated electric wire wound around the core, and electrodes disposed on the outer surface of the core, the end portions of the insulated electric wire being connected to the respective electrodes, the method including a low-pressure pressing step in which the insulated electric wire is pressed against the electrodes by a relatively low pressure at a temperature high enough to melt the insulating coating of the insulated electric wire; and a high-pressure pressing step performed following the low-pressure pressing step, in which the electric wire is pressed against the electrode by a relatively high pressure so that the electric wire becomes flat and so that the flattened electric wire is embedded in the electrode.
The temperature in the high-pressure pressing step may be set to a value higher than the melting point of the top portion of the electrode and lower than the freezing point thereof.
The high-pressure pressing step may be performed at a temperature either lower or higher than the temperature at which the low-pressure pressing step is performed.
Preferably, the electrode of the coil device includes a base layer preferably formed via coating and baking an electrically conductive paste such as a silver-filled paste, a solder barrier layer disposed on the surface of the base layer using a metal material such as Ni having high resistance to erosion by solder, and an easy-soldering layer disposed on the outer surface of the solder barrier layer using a metal material such as Sn or solder which is easy to solder.